In recent years, there have come into widespread use devices which subject an image to compression encoding by employing an encoding format for handling image information as digital signals, and taking advantage of redundancy peculiar to the image information with transmission and storage of high effective information taken as an object at that time to compress the image by orthogonal transform such as discrete cosine transform or the like and motion compensation. Examples of this encoding method include MPEG (Moving Picture Experts Group) and so forth.
In particular, MPEG2 (ISO/IEC 13818-2) is defined as a general-purpose image encoding format, and is a standard encompassing both of interlaced scanning images and sequential-scanning images, and standard resolution images and high definition images. For example, MPEG2 has widely been employed now by broad range of applications for professional usage and for consumer usage. By employing the MPEG2 compression format, a code amount (bit rate) of 4 through 8 Mbps is allocated in the event of an interlaced scanning image of standard resolution having 720×480 pixels, for example. Also, by employing the MPEG2 compression format, a code amount (bit rate) of 18 through 22 Mbps is allocated in the event of an interlaced scanning image of high resolution having 1920×1088 pixels, for example. Thus, a high compression rate and excellent image quality can be realized.
With MPEG2, high image quality encoding adapted to broadcasting usage is principally taken as a object, but a lower code amount (bit rate) than the code amount of MPEG1, i.e., an encoding format having a higher compression rate is not handled. According to spread of personal digital assistants, it has been expected that needs for such an encoding format will be increased from now on, and in response to this, standardization of the MPEG4 encoding format has been performed. With regard to an image encoding format, the specification thereof was confirmed as international standard as ISO/IEC 14496-2 in December in 1998.
Further, in recent years, standardization of a standard serving as H.26L (ITU-T Q6/16 VCEG) has progressed with image encoding, originally intended for television conference usage. With H.26L, it has been known that as compared to a conventional encoding format such as MPEG2 or MPEG4, though greater computation amount is requested for encoding and decoding thereof, higher encoding efficiency is realized. Also, currently, as part of activity of MPEG4, standardization for also taking advantage of a function that is not supported by H.26L with this H.26L taken as a base, to realize higher encoding efficiency, has been performed as Joint Model of Enhanced-Compression Video Coding. As a schedule of standardization, H.264 and MPEG-4 Part 10 (Advanced Video Coding, hereafter referred to as H.264/AVC) become an international standard in March, 2003.
Further, as an extension thereof, standardization of FRExt (Fidelity Range Extension) including a coding tool necessary for business use such as RGB, 4:2:2, or 4:4:4, 8×8DCT and quantization matrix stipulated by MPEG-2 has been completed in February, 2005. Thus, H.264/AVC has become a encoding format capable of suitably expressing even film noise included in movies, and has been employed for wide ranging applications such as Blu-Ray Disc (registered trademark) and so forth.
However, nowadays, needs for further high-compression encoding have been increased, such as intending to compress an image having around 4000×2000 pixels, which is quadruple of a high-vision image. Alternatively, needs for further high-compression encoding have been increased, such as intending to distribute a high-vision image within an environment with limited transmission capacity like the Internet. Therefore, with the above-mentioned VCEG (=Video Coding Expert Group) under the control of ITU-T, studies relating to improvement of encoding efficiency have continuously been performed.
Now, one factor that can be given why the H.264/AVC format realizes high encoding efficiency as compared to the conventional MPEG2 format or the like is employing an intra prediction method.
With the intra prediction method, the intra prediction modes of nine kinds of 4×4 pixel and 8×8 pixel block units, and four kinds of 16×16 pixel macro block units are determined regarding luminance signals. The intra prediction modes of four kinds of 8×8 pixel block units are determined regarding color difference signals. The intra prediction modes for color difference signals may be set independently from the intra prediction modes for luminance signals.
There are particular patterns for each intra prediction mode regarding how residual following such intra prediction is manifested.
As a method to eliminate such redundancy and further raise encoding efficiency, NPL 1 proposes the following method.
That is to say, intra image encoding processing is performed by normal H.264/AVC format using training signals in offline processing beforehand, orthogonal transform such as Karhunen-Loéve transform or the like is performed for each intra prediction mode as to each block, and optimal transform coefficients are calculated.
Then, in the actual encoding processing, processing using orthogonal transform coefficients optimized for each mode by the aforementioned Karhunen-Loéve transform are used instead of the orthogonal transform stipulated by the H.264/AVC format.
Also, NPL 2 proposes a method of combining the aforementioned intra prediction with inter prediction.
That is to say, with NPL 2, difference information is generated as to motion vector information obtained in inter prediction not only for a current block but also for neighboring pixel values around the current block. Performing intra prediction between the difference information relating to the current block, and the difference information relating to neighboring pixels, generated in this way, generates second order difference information. The generated second order difference information is then subjected to orthogonal transform and quantization, and output downstream along with a compressed image.
Thus, encoding efficiency is further improved.
Also, as described above, the macro block size is 16×16 pixels with the H.264/AVC format. However, a macro block size of 16×16 pixels is not optimal for large image frames such as UHD (Ultra High Definition; 4000×2000 pixels) which will be handled by next-generation encoding methods.
Accordingly, NPL 3 and so forth propose enlarging the macro block size to a size of 32×32 pixels, for example.